1. Field of the Invention
The invention relates to a control circuit and method for controlling a gain applied to a composite video signal, and more particularly, to an analog-to-digital converting system having a hysteretic control circuit and related method for jointly adjusting a gain factor of an amplifier and a reference voltage of an analog-to-digital converter to control the gain applied to the composite video signal.
2. Description of the Prior Art
Since the introduction of the Advanced Television Systems Committee (ATSC) standard, great effort has been expended toward perfecting a digital TV system. For compatibility, a digital TV system designed for decoding a digital TV signal that complies with an ATSC specification must also be capable of decoding a composite video signal that complies with specifications of the traditional analog TV systems, such as: the National Television System Committee (NTSC) standard or the Phase Alternate Line (PAL) standard. In this way, compliance with these various specifications offers a user the ability to view TV programs using the same equipment regardless of the specification by which information is being transmitted by a digital TV signal or an analog TV signal.
In advance of performing any signal processing on the incoming composite video signal, an amplitude range of the composite video signal must be properly adjusted such that the following signal processing stages of a video decoding circuit can decode the adjusted composite video signal more accurately. As known to those skilled in this art, the amplitude range of the composite video signal is crucial to a decoding procedure of the composite video signal since both chrominance and luminance information of the composite video signal is related to the amplitude of the composite video signal. Therefore, controlling the amplitude range of the composite video signal is critical to the performance of decoding the composite video signal.
In the prior art, an amplifier is usually adopted for controlling the amplitude of a composite video signal to be limited to a certain pre-determined range. Then an output signal of the amplifier, an adjusted composite video signal, is further digitized by an analog-to-digital converter (ADC). However, this may prevent the ADC from digitizing the adjusted composite video signal in an optimal manner because a reference voltage value of the ADC may be much greater than a maximum amplitude of the adjusted composite video signal. That is to say, if the pre-determined range set to the amplifier cannot be tuned to be only slightly lower than the reference voltage of the ADC then the ADC cannot be fully utilized to improve the performance of a decoder used to decode the composite video signal due to a fact that the quantization error of the ADC becomes greater compared to the pre-determined range of the amplifier. This results in a poor resolution of analog-to-digital converting the composite video signal.